A standard single-control mixing valve (see U.S. Pat. Nos. 4,621,659 and 4,540,023) used in a single-lever faucet has a normally cylindrical housing centered on an axis and a valve plate closing the bottom of the housing and formed with a hot-water inlet port, a cold-water inlet port, and an outlet port. A control plate sits flat atop this valve plate and is formed with a cavity that normally overlies the outlet port and that can also be brought into registration over the inlet ports, either singly or jointly. A lever pivoted on the housing is connected to this control plate to slide it atop the valve plate, thereby determining the amount of overlap of the control-plate cavity with the inlets to determine the amounts of hot and cold water to be shunted through this cavity to the outlet port.
As described in German patent document 1,291,957 of W. E. Blodgett (based on U.S. application Ser. No. 543,456 filed 22 Mar. 1966) it is standard to make the valve and control plates both of a hard ceramic, and in German Utility Model 8,606,471 of W. Heinzel a system is described where instead of a solid ceramic body a glass plate with a hard ceramic coating is used. In both arrangements there is ceramic-to-ceramic contact at the sliding interface between the valve and control plates. Even though the contact surfaces are always machined to high tolerances so as to be perfectly planar, there is substantial friction between them, making the valve stiff to operate.
This stiffness is countered in part by coating the interface surfaces with a heavy lubricant grease. The result is, of course, a substantial lessening both of static and dynamic friction, but only so long as the grease is present. With time, especially when in the presence of hot water, the grease is carried away so that the valve becomes stiff to operate. Regreasing the contact surfaces is a difficult job entailing dismantling the entire valve after shutting off the water, so the end user of the valve must normally put up with stiff operation once the valve is no longer new.